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Our transcription: The carbon in air and water is made up of three different isotopes. One of these, carbon-14, is radioactive. As part of the atmosphere carbon-14 is absorbed by living plants and animals. In this way, the carbon in all living organisms contains the same proportion of carbon-14 to non-radioactive carbon. When the organism dies, it stops taking in any new carbon, and the amount of carbon-14 inside it gradually decreases as it decays. It is this shrinking proportion of carbon-14 that tells us how long ago the organism died.
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